This invention relates to imaging apparatus of the kind comprising a radiation source and a detector which are moveable relative to a subject; a collimator for adjusting the effective width of an imaging beam generated by such apparatus; and a method of operating such apparatus.
Imaging apparatus which utilizes an x-ray source and an associated detector is known, in which the x-ray source and detector are moved relative to a subject in a scanning process, in order to generate a composite x-ray image of the subject. Apparatus of this kind is described in International patent application no. WO 00/53093, for example. In the apparatus described in the abovementioned patent application, an x-ray source is mounted on one end of a C-shaped arm and emits a low-dose collimated fan beam of x-rays. Mounted on the opposite end of the C-arm is an x-ray detector unit, comprising scintillator arrays optically linked to charge coupled devices (CCDs). An image is obtained by linearly moving the C-arm over the length of a subject or patient with the x-ray source active, while continually reading the output of the detectors. The principle of operation of such apparatus is commonly referred to as linear scanning.
In apparatus of this kind, a number of possibly conflicting parameters affect the characteristics of the final image. Firstly, in order to increase the resolution of a linear slit scanner, it is usually desirable to reduce the width of the imaging beam in the scanning direction as far as possible. This requires the use of a beam limiting device, which is wasteful of x-ray energy and requires a relatively higher x-ray generation capacity than a comparable full field system. A second consideration relates to a trade-off between spatial resolution and contrast resolution. In the example apparatus referred to above, the output of individual pixels in the CCDs can be combined or “binned”, which lowers the spatial resolution of the resultant image but enhances the signal to noise ratio and thus the contrast resolution of the image. Depending on the nature of the image required, contrast resolution may be a more important image characteristic than spatial resolution in certain cases.
In the light of the above considerations and others, it is an object of the invention to provide imaging apparatus, and a method of operating such apparatus, which enable optimization of a selected characteristic of the image signals generated.